This invention relates to mobile telephone communication systems using so-called small radio zone techniques, and more particularly to channel exchange systems in such communication systems.
In a mobile telephone communication system of this kind (for example that of U.S. Pat. Nos. 4,144,409, U.S. Pat. No. 4,144,496 or U.S. Pat. 3,898,390), the service area is divided into many small radio zones, in each of which is established a base station. More than 2 such small zones constitute a control zone, in which is set up a mobile control station, which in turn is connected to the ordinary telephone service network. Between the ordinary telephone service network and the mobile control station is provided a mobile telephone center having the function of converting the signals used in the radio section into the form of signals used in the existing exchange network. However, such mobile telephone centers are sometimes installed in exchange stations housing switchboards for the ordinary telephone service network as well.
Since the service area of such mobile telephone communication system, as stated above, is divided into a large number of small zones, a mobile unit may move from one small zone to another in the course of the same conversation. In such an instance, it is necessary to avoid interruption of the conversation when the mobile unit passes the boundary between zones by switching from the speech channel of the first zone to that of the second zone. This channel switching, by prior art, is accomplished in the following manner: the crossing of the boundary between zones by a mobile unit in the course of a conversation is detected by the level detecting circuit of the speech channel receiver at the base station, sensing a drop in received carrier wave level below the predetermined level; and the level degradation signal is transmitted to the mobile control station, which, upon receipt of this signal, transmits level monitoring demand signals to base stations around the base station whose signal level has degraded; each base station having received this demand signal switches its level monitoring receiver to the speech channel designated by the demand, monitors the received carrier level and reports the result of monitoring to the mobile control station, which, comparing these level data from the radio base stations, selects the station having the maximum level and assigns a new channel; after the selection of the new channel, a channel exchange instruction signal is transmitted to the mobile unit through the channel hitherto used for the conversation; and the mobile unit, assigned a new channel, switches over to this new channel and continues the conversation thereon. For further details on such systems, reference is made to Kamata et al., "800 MHz Band Land Mobile Telephone Radio System," and Nagatsu et al., "Base Station Radio Equipment for 800 MHz Band Land Mobile Telephone System," both of Review of the Electrical Communication Laboratories Nippon Telegraph and Telephone Public Corporation, Volume 25, Numbers 11-12, November-December, 1977.
Such a channel exchange system has a disadvantage in that the mobile unit may erroneously receive the channel exchange instruction signal and interrupt the conversation, because the instruction signal is transmitted over the previous speech channel on which the received voltage level has already degraded. Furthermore, each base station would require a carrier level monitoring receiver which can tune in to every speech channel at the disposal of surrounding radio base stations, and such carrier level monitoring receivers are expensive and therefore undesirable.
Accordingly, the objective of the present invention is to provide a highly reliable and moreover relatively inexpensive mobile telephone channel exchange system which is free from the disadvantages stated above.